These two start from different ends. Opik is a general evaluation platform with datasets, experiments, configurable judges and prompt versioning, free to self-host; Raindrop watches live traffic, finds recurring failures and compares changes on real users. Whether your agent is in production yet often decides it.[1][4]
Opik is an open-source (Apache-2.0) platform for tracing and evaluating agents, before release and in production; Raindrop is a hosted platform for monitoring agents already in production, which classifies events with custom signals and groups recurring failures into issues. Opik gives you judges, datasets, prompt versioning and free self-hosting. Raindrop has no prompt versioning or hosted offline eval runner, and builds its classifiers with its own models.
| Decision factor | Opik | Raindrop |
|---|---|---|
| Ownership | Comet ML, independent; reports $70M in total funding | Independent; Series A led by CRV in September 2026, $50M in total |
| Centre of gravity | Evaluation: datasets, experiments, judges, online rules | Production monitoring: signals, issue detection, live-traffic experiments |
| Judges | 30+ built-in metrics, custom judge prompts, Python metrics | Signals drafted from a plain-language description by Raindrop's own models |
| Offline evals | Hosted datasets, experiments and Test Suites | Local Workshop tool; Simulations on pull requests in early access |
| Prompt versioning | Prompt Library with environment-tagged versions | None documented |
| Self-hosting | Free, Apache-2.0; Kubernetes for production | Enterprise VPC deployment in beta, select partners |
| Free tier | Free Cloud: 25k spans a month, 60-day retention | Hobby: 1,000 events a month, 14-day retention |
| First paid tier | Pro Cloud: $19 a month, 100k spans | Pro: $299 a month plus $0.003 per event |
| Capability | Opik | Raindrop | Prefactor |
|---|---|---|---|
| Observe | |||
| LLM call tracing | ✓ | ✓ | ✓ |
| Multi-step agent tracing (tool calls, nested spans) | ✓ | ✓ | ✓ |
| Cost tracking | ✓ | ✓ | ✓ |
| OpenTelemetry support | ✓ | ✓ | ✓ |
| Evaluate | |||
| Offline evals (datasets, experiments) | ✓ | Partial | ✓ |
| Online evals on production traffic | ✓ | ✓ | ✓ |
| LLM-as-a-judge scoring | ✓ | Partial | ✓ |
| Code-based evaluators | ✓ | Partial | ✓ |
| Human review and annotation | ✓ | Partial | ✓ |
| Prompt versioning and playground | ✓ | — | — |
| Act on findings | |||
| Runtime controls on live agent actions | Partial | — | ✓ |
| Gateway or proxy in the request path | Partial | — | — |
| Licence and deployment | |||
| Open source | ✓ | Partial | Partial |
| Self-hosting | ✓ | Partial | Partial |
Opik covers the full loop. You build datasets, run experiments and Test Suites with plain-text assertions, compare prompt versions in a Prompt Library, and then apply online rules that score sampled production traffic. It suits a team that wants the same judges in development and in production.
Raindrop is built for agents that already have users. Signals are classifiers that run on every new event and can backfill 90 days of sampled history; Issue Detection groups recurring failures; Experiments compare signal rates between cohorts of live traffic, such as two prompt versions. A Triage Agent investigates issues in the web app, in Slack or over MCP.
Raindrop's pre-release tooling is lighter. Workshop is an MIT-licensed local debugger where a coding agent turns failures into code-level evals and reruns the agent until assertions pass. Simulations, which replay production traffic against a proposed change on each pull request, are in early access.
Both track cost. Raindrop's Costs page shows model spend by model, provider and project, and its Experiments report median cost per event. Opik tracks token usage and model cost on traces, and its Cost Intelligence extends this to coding agents such as Claude Code and Codex.
In Opik you choose and tune the judge: built-in metrics such as hallucination, answer relevance and task completion, custom judge prompts, an 'Agent as a Judge' that reads traces with tools, or Python metrics that subclass BaseMetric. Since July 2026 you can cap spend per evaluation.
In Raindrop you describe the behaviour in plain language, label sample events as Match or Not Match, and Raindrop drafts a classifier. Its rd-signal-2 system, released in August 2026, combines generated filter code, a task-specific classification head and an in-house reasoning model for unclear cases. You cannot bring your own judge prompt or model. That trade suits teams who want classification without writing judges; it does not suit teams who must control exactly how a score is produced.
There is some overlap on the debugging side. Opik's Ollie assistant analyses traces and writes fixes with test cases, which covers ground similar to Raindrop's Triage Agent and Workshop, from a different starting point.
Opik's core platform is Apache-2.0 and free to self-host, with every feature except user administration. Raindrop Cloud is proprietary; self-hosting is in beta as an Enterprise VPC deployment for select partners. Both accept OTLP traces over HTTP and neither accepts gRPC.
Opik offers guardrails your code calls as a blocking check, with English-only PII and topic checks. Raindrop documents no blocking or guardrails: it analyses events after they arrive and alerts, and its PII Guard add-on redacts data at ingestion rather than controlling agent actions.
Framework coverage overlaps, but maturity differs. Opik lists more than 30 integrations, with SDKs for Python, TypeScript, Java and .NET. Raindrop's SDKs cover TypeScript, Python and Go, with Rust and Java in beta, and most of its framework integrations are labelled beta.
As of 20 October 2026, Opik self-hosting is free with unlimited spans. Free Cloud covers 25k spans a month, and Pro Cloud is $19 a month for 100k spans, then $5 per 100k, with 60-day default retention.
Raindrop's Hobby plan is free for 1,000 events a month, after which ingestion stops until the next month, with one custom signal and 14-day retention. Pro is $299 a month plus $0.003 per event up to 1M events and $0.002 after that, with 10 custom signals and unlimited retention. PII Guard is a separate add-on from $80 a month. The entry prices differ by an order of magnitude, and so does what each plan includes. See Opik's pricing and Raindrop's plans.
Raindrop's Pro plan also includes 500 Triage Agent messages a month.
We sell the layer this section describes. Read it with that in mind.
Stated plainly, since Prefactor publishes this page: Prefactor is an agent evaluation platform that scores each step of a run and acts on what it finds through runtime policies on tool calls and data access, approval holds for risky actions, and a kill switch for a run, an agent or a team. Raindrop detects failures after the fact and Opik's guardrails run only where your code calls them; neither holds an agent action for approval. Prefactor has no prompt playground or prompt registry and its platform is hosted, so for prompt iteration or free self-hosting Opik is the better pick, and for failure discovery across large volumes of user traffic Raindrop is built for that job. See the full comparison of 15 platforms.
Reviewed against public product and documentation pages on October 20, 2026. If a vendor has changed a feature, product name, or positioning since then, send a correction and we will update it. Numbered source links in the page body point to the ordered sources below.
Prefactor scores each step of every agent run and can hold or stop an action when a score says it should not happen. Start on the free tier: 5,000 spans a month, no credit card required.
Prefactor helps teams observe, evaluate, and improve their AI agents in production, across every framework and provider.